1. Field of the Invention
This invention relates to improvements in an exhaust gas recirculation system for an internal combustion engine of a vehicle such as an automotive vehicle, and more particularly to a device for diagnose abnormality of the exhaust gas recirculation system such as abnormality of an EGR valve.
2. Description of the Prior Art
Automotive internal combustion engines are equipped with an exhaust gas recirculation (EGR) system as a measure for exhaust emission control, in which a part of exhaust gas is recirculated through an EGR passage back to engine cylinders each having a combustion chamber. In the EGR system, control of the amount of exhaust gas to be recirculated back to the combustion chamber is carried out by an EGR valve operated in accordance with the operating condition of the engine or the vehicle. In such an EGR system, it is necessary to operate the EGR system without trouble by watching as to whether any abnormality arises or not in the EGR valve and the EGR passage through which the recirculated exhaust gas flows.
Such an exhaust gas recirculation system is disclosed, for example, in Japanese Patent Provisional Publication No. 61-182450. In this EGR system, exhaust gas recirculation is carried out by opening the EGR valve under the action of intake vacuum generated in the vicinity of a throttle valve of an engine and introduced into a vacuum chamber of the EGR valve. Additionally, a temperature sensor is disposed downstream of the EGR valve to detect the temperature of the recirculated exhaust gas, thereby achieving diagnosis of abnormality of the EGR system such as clogging of the EGR passage and failed operation of the EGR valve.
However, difficulties have been encountered in such an EGR system in which since the temperature sensor is disposed on the downstream side of the EGR valve taking account of preventing thermal damage of the temperature sensor, the above-mentioned abnormality diagnosis can be precisely carried out when a large amount of exhaust gas is recirculated, but cannot be carried out when a small amount of exhaust gas is recirculated. In other words, in case the amount of the recirculated exhaust gas is smaller, the recirculated exhaust gas has been cooled before reaching the temperature sensor, so that there is nearly no difference in temperature between the upstream and downstream sides of the EGR vlave. This makes difficult to precisely detect clogging of the EGR passage, failed operation of the EGR valve and the like.
It is to be noted that such an arrangement that the temperature sensor is disposed downstream of the EGR valve has been employed in the background of carrying out an exhaust gas recirculation with a large amount of recirculated exhaust gas in order to achieve exhaust emission control, in which consideration has been made to prevent thermal damage of the temperature sensor due to the large amount of the recirculated exhaust gas. Employment of such an arrangement is based on a standard of combustion performance of engines at the beginning of development of the EGR system. Thereafter, under development of a variety of combustion control techniques of engines, improved exhaust emission control has recently become possible without a large amount of recirculated exhaust gas, by using engine controls other than exhaust gas recirculation.
In view of the above, the inventors of the present application have found that the temperature sensor cannot be thermally damaged even in case of being disposed upstream of the EGR valve, in an engine in which a relatively small amount of exhaust gas is recirculated back to the combustion chambers. This has demonstrates that precise diagnosis of abnormality of the EGR system such as of EGR valve will be carried out even if the amount of recirculated exhaust gas is smaller.